Flotation separation of cryolite and fluorite



Dec. 20, 1960 RAW DRY CRYOLITE ORE L. E. GOLDENHAR ETAL Filed May 9, 1956 QUARTZ AND SIDERITE COLLECTOR WITHOUT NCIF PULPING WATER WITHOUT NCIF FLOTATION CELL QUARTZ AND SIDERITE- RICH FROTH DRY /ORE RICH IN C YOLITE ANDIFLUORITE DRYING CRYOLITE AND FLUORITE-RICH UNDERFLOW FLUORITE PULPING WATER COLLECTOR CONTAINING NcIF FLUORITE-RICH FROTH FLOTATION CELL RYOLITE-RICH UNDERFLOW WITH NGF SID/ERITE C LLECTOR RAw DRY FLUORITE CRYOLITE ORE COLLECTOR sIDERITE-RICI-I FROTH I PULPING WATER FIRST BANK OF CONTAINING NCIF UNDERFLOW RICH CRYOLITE QUARTZ AND FLUORITE FLOTATION CELLS QUARTZ AND FLUORITE-RICH FROTH CRYOLITE- RICH UNDERFLOW LEON ARD E GOLDENHAR IN V EN TORS.

ISADORE MOCKRIN FRANCIS M. DOORLEY, JR.

ATTORNEY United rates Patent 'ce FLOTATION SEPARATION OF CRYOLITE AND FLUORITE Leonard E. Goldenhar, New Kensington, Isadore Mockrm, Philadelphia, and Francis M. Doorley, In, Tarentum, Pa., assignors to Pennsalt Chemicals Corporation, a corporation of Pennsylvania.

Filed May 9, 1956, Ser. No. 583,622

15 Claims. (Cl. 209-166) The present invention relates to the beneficiation of cryolite ore, and more particularly to the separation of fluorite from cryolite by froth flotation.

Cryolite (sodium fluoaluminate) is an essential material in the production of aluminum by electrolytic reduction processes wherein it is used as the electrolyte and solvent for alumina. It is also of value for its fluorine content in the ceramic and glass industries.

Cryolite has been mined from natural deposits, principally in Greenland, for many years to supply these industries. In nature, cryolite is usually associated with a number of gangue minerals, chief among which are siderite, quartz and heavy metal sulfides presently being removed by froth flotation.

In the removal of siderite and quartz from cryolite ores, the generally accepted method has employed two banks of flotation cells to which the pump is fed at a pH between about and 6.8. In the first bank the pulp is agitated in the presence of a collector such as pine tar oil and the siderite is removed in the foam. Any heavy metal sulfides that may be present are also removed in this step. The underflow passes directly to the second bank, where the quartz is removed, using a collector such as caustic and oil-gas mixture and lead nitrate, and the purified cryolite emerges as the underflow and is centrifuged and dried. Each bank of cells is arranged in conventional manner to provide a certain amount of recycling of both the tailings and the concentrate in order to improve yields. In this process any fluorite that may be present in the ore remains in the underflow with the cryolite since this froth flotation process is incapable of causing any appreciable separation of the fluorite.

Since the cryolite ore has in the past contained only small amounts of fluorite, the removal of the quartz and siderite from the cryolite ore by froth flotation has been suflicient beneficiation to secure cryolite concentrates of acceptable purity, specifications generally requiring a cryolite content of at least 96.0%. Trace amounts of other minerals have largely been ignored, since their presence caused no trouble.

However, in recent years the naturally occurring cryolite deposits have become gradually depleted, with the result that the cryolite content is dropping and the gangue minerals are increasing in the ore now being mined. In particular, fluorite has recently become a major impurity, the average fluorite content having increased from trace amounts to about 2%, with occasional batches containing up to 9% Prior to the present invention, it was considered impossible to separate fluorite and cryolite by froth flotation since, in spite of repeated attempts to float specifically either one or the other, the two minerals, cryolite and fluorite seemed to act as a single entity in the flotation cells, and the fluorite was always recovered intact in the cryolite fraction. Thus in practice if an ore containing 67% cryolite and 1.0% fluorite were upgraded by removal of the siderite and quartz to the point that the cryolite content was 96.0% cryolite, the fluorite con- 2,965,233 Patented Dec. 20, 1960 tent would correspondingly increase to 1.4%. Since the upgraded cryolite also contains small amounts of other impurities not removed by flotation, the net result of the increase of fluorite content is that a definite limit is placed on the purity of cryolite that can be obtained. For this reason, it was an accepted fact that cryolite of suflicient purity could not be produced from ore containing fluorite in amounts greater than about 1.25%, and high fluorite ores could not be processed. The only alternatives for fluorite removal known were either to resort to differential grinding of the ore or to hand-picking the fluorite from the crude ore, both of which methods are uneconomical.

To facilitate a clear understanding of the invention, reference may be made to the accompanying drawing wherein:

Figure 1 is a self-explanatory flow diagram involving two flotation steps; and

Figure 2 is a self-explanatory flow diagram involving two flotation steps and an intermediate drying step.

It has now been discovered that fluorite can be separated from cryolite by froth flotation provided, when the pulp is being prepared, the dry ore is mixed with an aqueous solution of sodium fluoride. The pulp so prepared is then fed to flotation cells, where it is agitated in the presence of a collector of the fatty acid type to form a froth rich in fluorite, the cryolite remaining in the underflow. In separating the fluorite a water solution of sodium fluoride is prepared and this solution is then used to make the pulp together with substantially dry ore. If the sodium fluoride is added to the pulp after the ore has been slurried in untreated water, substantially no separation will occur.

Furthermore, in accordance with the present invention, the concentration of sodium fluoride in the make-up water must be in the range of l to 9 grams per liter if separation is to be obtained. In the preferred practice, the concentration should be between 1.5 and 7 grams per liter. The percentage of the total fluorite present that may be separated from cryolite in a given flotation is dependent on the amount of sodium fluoride present in the water. This percentage has a maximum value when the sodium fluoride concentration is 3 grams per liter, and decreases at greater or less concentrations. It is therefore preferred to maintain the sodium fluoride concentration at values as close to 3 grams per liter as may be practicable.

It has moreover been ascertained that the concentration of sodium fluoride is critical with respect to the amount of water used to make the pulp, and not to the amount of ore. Thus the pulp may have a solids content anywhere within the conventional range of about 25 to 45%, but the fluoride level in the make-up water must be maintained within the range specified above to obtain separation of the fluorite in the subsequent float.

The flotation of the fluorite, after the pulp has been prepared as described, may then be carried out at a pH of between about 4.5 and 6.5, the best results being obtained in the pH range 5.8 to 6.0. Since various batches of ore will generally give varying pH values when slurried in water, it is usually necessary to adjust the pH to the desired value with conventional reagents. Nitric acid is among the mineral acids generally used for acidifying and caustic soda is an example of a suitable alkalizer. The pH regulators may be added directly to the flotation cells or in a preliminary conditioning step, as desired.

Suitable collecting agents for the fluorite include fatty acid collectors such as oleic acid or linoleic acid, or sodium oleate or preparations such as the Neofats made by Armour & Co., Aliphat 44E made by General Mills, etc., all of which contain some oleic acid. The preferred collector is a mixture of fatty acids containing about 40% oleic acid. The collector may be used in amounts .of about 0.5 to 4.0 pounds per .ton of ore. It is preferred to add the collector in several small increments rather than adding .the total amounts in one;portion, since -more complete separation and maintenance of higheryielids are obtained in this manner.

In practicing the present invention, where ore'com taining essentially cryolite and fluorite is to be floated to secure either the cryolite or the fluorite valuestherein, the following general procedure is;carried out. The-dry ore is crushed and ground sufficiently to pass a 60 mesh screen (the number representing openings per linear inch). Undersize material is not detrimental to .the process, feeds containing as much ,as 40% .minus 625 mesh material having been floatedsuccessfully.

The ground, dry ore is then made into a pulp. The water for making the pulp should contain from 1 to 9 grams sodium fluoride per liter, and :preferably 3 grams. (Warm water, at temperatures of about 105 to 115 =F., is generally preferred in flotation processes since itimproves the mobility of the collectors, which usually belong to the class of relatively high melting organic compounds that produce foam.) The pulp has preferably a solids content of about 40 to 42%. The pH of the pulp is adjusted to about 6 as the pulp is fed to the flotation apparatus. The apparatus may, for example, be arranged in a bank of flotation cells according to the, scheme shown in Table I.

TABLE I Typical flow arrangement for flotation cell bank Cell N Source of Feed Froth. to- Umtlerflow Froth from cell No. 2 Fluorite Con- Cell No. 2.

centrate.

Fmth from cell N0. 3 Cell No. Cell No. 3.

Fresh pulp Cell No. Cell No. 4.

Underfiow from cell No. 8 Cell No. Cell No. 5.

Undcrfiow from cell No. 4. Cell No. 6.

Undertlow from cell No. 5 Cell No; .'7.

Underfiow from cell No. 6 Cell No. 8. Underflow from cell No. 7 Cell No. 4- Cryolite Goncentrate.

In such a scheme, the pulp enters the third cell of the bank, and the fluorite collection agent is added to one or more cells and preferably to 2 to 4 successive cells, starting with the third in amounts up to about 1.0 pound per ton to each cell. The first two cellsare thus reserved for stripping the froth of any remaining cryolite, and the last two cells similarly remove the last traces 0f :fluorite. The underflow from the last cell contains the cryolite concentrate and the froth removed from the first cell contains the fluorite concentrate. The two concentrates, or either concentrate that it is desired to recover, are then centrifuged and dried in the conventional manner.

In this process it is advantageous to employa closed water system, wherein the water removed from the puri fied product is returned to the make-up water supply since by this expedient considerable savings can berealized through conservation of reagents and heat.

The following example, while-:not intended to limit the scope of the invention, is illustrative of the benefit derived from the practice thereof.

EXAMPLE I Two comparative flotations were :cartiedout using :a single flotation cell. F or'these flotations the ore; feedconsisted of 90.97% cryolite, 0.38% siderite, 0.70% quartz and 6.57% fluorite. In each float the pH was adjusted to 5.5 and oleic acid, added in four small amounts, was used as the collector. In the first float, no sodium-fluoride was used. The cryolite concentrate (underflow) analysed 90.18% cryolite and 8.71% fluorite. lln thesecond float, sodium fluoride was dissolved in .themake-up water to a concentration of 3 grams per liter, and azslurryrmade from this solutionand dry; ore. 'Iheresulting concentrate assayed 94.15% cryolite and only 3.57% fluorite. It is -apparentjrom thiscomparison that only in the second float was a differential flotation of fluorite obtained.

Of great practical importance is the further discovery that the process of the present invention may be carried out in the same cell banks and at the same time as the quartz and siderite are 'removed in the purification of cryolite ore. This can be done only under-certain conditions, governed by the effect of the sodium fluoride on the reagents present for separating the siderite and quartz. These reagents are present in'rather delicate balance,-and while a certain .amountof sodium. fluoride-can be tolerated in the system, amounts in excess of about 1.5 grams per liter cause these reagents to foam excessively or otherwise to decreasetheeifectiveness of the quartz and siderite removal or decrease the yield of cryolite. It is therefore impractical to use sodium fluoride concentrations greater than 1.5 grams per liter in the make-up water where the fluorite is to be removed at the same time "as the othenimpurities. .Since the optimum concentration of sodium fluoride, corresponding to removal of the highest percentage of the total fluorite present, is not tolerated by the combined system, a smaller amount must be used with the result that a relatively smaller amount of fluorite will be removed in a single flotation thus limiting the initial amount of fluorite permissible. It has been ascertained that in .the combined flotation of quartz, siderite and fluorite in accordance with the present invention, concentrates containing greater than 97.0% cryolite may be obtained if this initial amount of fluorite in the ore feeds does not exceed 2.2%. Thus, the combined flotation, while actually removing only a relatively small percentage of the fluorite, performs the more important function of preventing the carry-over of the fluorite content into the cryolite concentrate, and makes possible the attainment of cryolite sufficiently pure to meet specifications from practically all the ore previously considered unfit forthis purpose. Furthermore, while the convenience of performing the separation of the three contaminating minerals (siderite, quartz, and fluorite) in a single step is apparent, and while this procedure may be employed successfully whenever the fluorite content is less than 2.2%,, there remains in addition the advantage that fluorite maybe removed, even when in excess of 2.2%, by employing a separate flotation step for the fluorite alone in accordance with the procedure herein above described. When using two separate flotation steps, viz., one ,for the quartz andsiderite and one for the fluorite, after separating the quartz and siderite from the ore by conventional flotation procedures in the absence of sodium fluoride as previously described, the ore, now containing onlysmall amounts of the latter two. constituents is first substantially dried and then mixed with the sodium fluoride solution to form a pulp for the second flotation step in which the fluorite is removed. Figure 2 of the accompanying drawings illustrates in a self-explanatory flow diagram .such as procedure involvingtwo separation flotation steps with intermediate drying.

In practicing the simultaneous separation, which is illustratedin Figure 1 of the drawings by a self-explanatory flow diagram the ore, containing fluorite, quartz and siderite as impurities, is pulped with water containing from 1 to 1.5 grams sodium fluoride per liter, the water preferably being Warm. The pH is adjusted to within the;range:5.0.to 6.5. The pulp thus prepared isied to the flotation cells which, as described above, generally consist of .a double bank of cells arranged according to the scheme shown in Table I. The siderite is removed in the first bank through agitation of the pulp in the presence of any suitable collector, pine tar oil generally being preferred. The-presence of sodium fluoride in the water has little if any eifecton this separation. The underflow, containing cryolite, quartz-and fluorite passes to ,thesec- 0nd bank, where the quartz and fluorite collectors are added with agitationgof the-pulp ,to form afrothrichain quartz and fluorite; .:It is preferable; to add 'theyfluorite collector to several successive cells rather than all at once to a single cell. The cryolite is removed in the underflow of each of these cells and the final cryolite concentrate is recovered in the underflow of the final cell.

Although the generally known collecting agents for quartz may be used in the simultaneous separation just described, it has been found that the best results are obtained Where the collector used is Amine (described by its manufacturer, the Geigy Chemical Corporation, as a mixture of high molecular weight teritary amines having a melting point of C.).

Example II illustrates the excellent cryolite concentrates obtained from high fluorite ores when the process of the present invention was introduced into the regular cryolite beneficiation operation.

EXAMPLE II Ore #1 Ore #2 Ore #3 Percent Percent Percent 97. 7 97. 26 97. 60 0.03 0.06 0.05 0. 34 0.25 0.48

Having described our invention, we claim:

1. The method of separating fluon'te from cryolite comprising mixing an aqueous solution of sodium fluoride containing 1 to 9 grams sodium fluoride per liter of water with a substantially dry finely divided mixture of cryolite and fluorite to form a pulp, agitating said pulp in the presence of a collector to form a froth rich in fluorite and an underflow rich in cryolite and separating said froth from said underflow.

2. The method of claim 1 wherein the pH of said pulp is maintained within the range of about 4.5 to 6.5.

3. The method of claim 2 wherein said collector is a mixture of fatty acids which contains at least about 40% oleic acid.

4. The method of claim 1 wherein said solution contains approximately 3 grams sodium fluoride per liter of water.

5. The method of separating fluorite from cryolite comprising mixing an aqueous solution of sodium fluoride containing about 1.5 to 7 grams sodium fluoride per liter of water with a substantially dry, finely divided ore containing substantial amounts of cryolite and fluorite to form a pulp, said pulp containing 25% to 45% solids, agitating said pulp in the presence of 0.5 to 4 pounds of a fatty acid collector per ton of ore to form a froth rich in fluorite and an underflow rich in cryolite, and separating said froth from said underflow.

6. The method of simultaneously separating quartz and fluorite from cryolite ore comprising mixing an aqueous solution of sodium fluoride containing 1 to 1.5 grams sodium fluoride per liter of water with said ore in a substantially dry, finely divided state to form a pulp, maintaining said pulp at a pH of between 5.0 and 6.5, agitating said pulp in the presence of a collector for quartz and a collector for fluorite to form a froth rich in quartz and fluorite and an underflow rich in cryolite, and separating said froth from said underflow.

7. The method of separating cryolite from ore containing in addition to cryolite appreciable amounts of siderite, quartz and fluorite comprising mixing an aqueous solution of sodium fluoride containing 1 to 1.5 grams sodium fluorite per liter of water with said ore in a substantially dry, finely divided state to form a pulp, maintaining said pulp at a pH of between 5.0 and 6.5, agitating said pulp in the presence of a collector for siderite to form a froth rich in siderite and an underflow rich in cryolite, quartz and fluorite, separating said froth from said underflow, subjecting said underflow to further froth flotation in the presence of a collector for quartz and a collector for fluorite, to form a froth rich in quartz and fluorite and a further underflow rich in cryolite, and separating said quartz and fluorite-rich froth from said further underflow.

8. The method of claim 7 wherein said collector for fluorite is a fatty acid.

9. The method of claim 8 wherein said collector for quartz is a mixture of high molecular weight tertiary amines having a melting point of about 5 C.

10. The method of claim 9 wherein said solution contains approximately 1.5 grams sodium fluoride per liter of water.

11. The method of separating cryolite from ore containing in addition to cryolite appreciable amounts of siderite, quartz and fluorite comprising preparing a pulp from the finely divided ore and water, agitating said pulp in the presence of collectors for siderite and quartz to form froth rich in siderite and quartz and an underflow rich in cryolite and fluorite, separating said froth and said underflow, removing substantially all the water present with the cryolite and fluorite in said underflow, adding to the resulting substantially dry mixture of cryolite and fluorite an aqueous solution of sodium fluoride containing 1 to 9 grams sodium fluoride per liter of water to form a second pulp, maintaining said second pulp at a pH of 4.5 to 6.5, subjecting said second pulp to froth flotation in the presence of a fatty acid collector to form a froth rich in fluorite and an underflow rich in cryolite and separating said froth from said underflow.

12. The method of claim 11 wherein said sodium fluoride is present in said solution in an amount of approximately 3 grams per liter of water.

13. The method of claim 3 wherein said fatty acid mixture is added to said pulp in small increments during froth flotation.

14. In the flotation separation of fluorite from cryolite, the step of mixing an aqueous solution of sodium fluoride containing one to nine grains of sodium fluoride per liter of water with a substantially dry mixture containing finely divided fluorite and cryolite to form a pulp, and then subjecting said pulp to froth flotation.

15. A method in accordance with claim '1 characterized by the use of a fatty acid collector for the fluorite.

References Cited in the file of this patent UNITED STATES PATENTS 2,497,863 Clemmer Feb. 21, 1950 FOREIGN PATENTS 557,804 Germany 1930 558,965 Germany 1932 702,343 France 1931 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2 965 23l December 20 1960 Leonard E, Goldenhar et al,

It is hereby certified that error appears in the above numoered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 31, for pump read pulp column 6 line 8, for "fluorite" read fluoride line 53 for "grains" read grams Signed and sealed this 9th day of May 1961.

(SEAL) Attestz- ERNEST W, SWIDER DAVID Lo [ADD Attesting Officer Commissioner of Patents 

1. THE METHOD OF SEPARATING FLUORITE FROM CRYOLITE COMPRISING MIXING AN AQUEOUS SOLUTION OF SODIUM FLUORIDE CONTAINING 1 TO 9 GRAMS SODIUM FLUORIDE PER LITER OF WATER WITH A SUBSTANTIALLY DRY FINELY DIVIDED MIXTURE OF CRYOLITE AND FLUORITE TO FORM A PULP, AGITATING SAID PULP IN THE PRESENCE OF A COLLECTOR TO FORM A FROTH RICH IN FLUORITE AND AN UNDERFLOW RICH IN CRYOLITE AND SEPARATING SAID FROT FROM SAID UNDERFLOW. 